The same problem was with all the browsers (the gliffy flash program didn't open and only a blank page was shown). IE just showed some debug information that a "}" was missing in the gliffy.js file.
I did some digging and found that this file is referenced in the page source that opens when the "edit diagram" -button is pressed:
<script language="JavaScript1.2" src="/download/resources/com.gliffy.integration.confluence:gliffy-macro-key/resources/gliffy.js"></script>
Don't know if this is why the gliffy flash program doesn't open, but I thought it was worth to mention it.
Oh, and yes "all" users are seeing this issue. (At least the four guys I asked to test this)
We're looking into this now.
-chris